Method of affixing ohmic contacts to ferromagnetic semiconductor bodies

ABSTRACT

GOOD OHMIC CONTACTS ARE AFFIXED TO THE SURFACES OF RARE EARTH CHALCOGENIDE FERROMAGNETIC SEMICONDUCTORS BY VAPOR DEPOSITING A 1:1 METALLIC COMPOUND WHICH IS A COMPONENT OF THE CHALCOGENIDE CRYSTALS ONTO THE SURFACE OF THE CRYSTAL. AS AN EXAMPLE, WHERE THE CHALCOGENIDE CRYSTAL HAS THE GENERAL FORMULA LN1-XLN&#39;&#39;XA, LN&#39;&#39;A IS VAPOR DEPOSITED ON THE CRYSTAL. THE DEPOSITED COMPOUND IS HEATED TO CAUSE IT TO PARTIALLY DIFFUSE INTO THE SEMICONDUCTOR, AND A CONDUCTING LEAD IS SOLDERED TO THE REMAINING OHMIC DOT.

April 13; 1971 F] HOL Z ERG 3.574575 METHOD OF AFFIXING OHMIC CONTACTS TO FERROMAGNETIC SEMICONDUCTOR BODIES Filed Sept. 19. 1968 12mm RT :Zmo 4.2K

AT TORNFY United States Patent 3,574,675 METHOD OF AFFIXING OHMIC CONTACTS T0 FERROMAGNETIC SEMICONDUCTOR BODIES Frederic Holtzberg, Pound Ridge, N.Y., assignor to International Business Machines Corporation, Armonk, NY. Filed Sept. 19, 1968, Ser. No. 760,866 Int. Cl. (3231f 17/00; C23c 13/02; C071? 11/00 US. Cl. 117-201 22 Claims ABSTRACT OF THE DISCLOSURE Good ohmic contacts are afiixed to the surfaces of rare earth chalcogenide ferromagnetic semiconductors by vapor depositing a 1:1 metallic compound which is a component of the chalcogenide crystals onto the surface of the crystal. As an example, where the chalcogenide crystal has the general formula Ln Ln' A, LnA is vapor deposited on the crystal. The deposited compound is heated to cause it to partially diffuse into the semiconductor, and a conducting lead is soldered to the remaining ohmic dot.

FIELD OF THE INVENTION This invention is directed to a method of aifixing ohmic contacts to rare earth chalcogenide bodies.

BACKGROUND OF THE INVENTION Rare earth chalcogenide compositions are becoming increasingly important in the fields of ferromagnetic semiconductors, thermoelectric devices, magneto-optical devices, magneto-resistive devices and the like. The importance of these materials have increased with the discovery that the Curie temperature thereof can now be controlled and can be expanded far above the liquid nitrogen temperature making their use much more convenient than in the past. Similarly, it has also been discovered that the resistivity of these materials can be varied over a range of 0 cm. A more detailed listing of rare earth calcogenides and the devices in which they find utility is set forth in the following commonly assigned patents and patent applications; 3,353,907; 3,371,042; 3,370,343; 3,370,924; and Ser. No. 411,525 and now Pat. No. 3,418,036; all assigned to the same assignee as this application.

A factor for limiting the exploitation of the potential uses of these materials in new devices, e.g., semiconductor devices, transducers and the like, has been the inability of producing good ohmic contacts thereon. In order to fully characterize the transport properties of the chalcogenides and exploit the interaction of magnetic, electrical and optical effects, it is necessary to develop methods of making ohmic contacts to single crystals having various levels of doping. The concept of making ohmic contacts on semiconductor materials by soldering with suitable alloys is well known. However, the prior art method and compositions for applying such contacts on the materials contemplated herein have not been met with any significant degrees of success. This is understandable, since there are no rules or principles for selecting alloys suitable for particular materials; one must therefor depend on empirical findings. Pat. No. 3,370,342, issued Feb. 27, 1968 and assigned to a common assignee, discloses one such empirical formulation, viz., a 60:40 Bi-Cd eutectic, for applying ohmic contacts to rare earth chalcogenides with varying success.

More recently good ohmic contacts have been made to rare earth chalcogenides with alloys consisting of a trivalent rare earth element and a conducting metal. These alloys are disclosed in a copending application Ser. No. 760,900, filed on Sept. 19, 1968 to R. I. Gambino 3,574,675 Patented Apr. 13, 1971 SUMMARY OF THE INVENTION Good ohmic contacts are afiixed to opposite surfaces of rare earth chalcogenide semiconductor compositions having the general formula Ln Ln' A by vapor deposit- 1ng thereon a 1:1 metallic rare earth chalcogenide having the formula LnA. Preferrably the LnA compound is a component of the rare earth chalcogenide semiconductor composition. The rare earth chalcogenides used in this invention are those described and contemplated in US. Pat. No. 3,371,041, to F. Holtzberg et al. and filed on June 11, 1964. For example, ohmic contacts are aflixed to a crystal of En Gd Se by vapor depositing 1:1 GdSe on the surface of the crystal and heating the crystal and its deposit to cause the deposit to partially diffuse therein. Because of the metallic nature of 1:1 GdSe, conductive leads e.g. (Cu), can be attached thereto.

OBJECT OF THE INVENTION An object of the invention is to provide a method of aflixing good ohmic contacts to semiconducting rare earth chalcogenide crystals wherein the ohmic contact is prepared from a compound which is a component of said semiconducting rare earth chalcogenide.

The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention.

The figure is a graph showing the IV characteristics of ohmic contacts prepared by the method of this invention.

DESCRIPTION OF PREFERRED EMBODIMENTS According to this invention, good ohmic contacts are made on a rare earth chalcogenide of the type where Ln is a first rare earth element, Ln is a trivalent rare earth element and different from Lu, and A is a chalcogen. These materials are prepared from a 1:1 ferromagnetic semiconductor e.g. (EuSe) and a trivalent rare earth metallic compound, e.g. (GdSe). The materials are characterized and a method for their preparation is disclosed in the above mentioned US. Pat. 'No. 3,371,041.

1:1 trivalent rare earth chalcogenide compositions may beprepared by the method described in the publication, Rare Earth Research, The McMillan Co., New York, N.Y., 1961, at page 135. The elements are separated in a sealed evacuated quartz enclosure. The chalcogen is transported in the 'vapor phase to the rare earth metal at a temperature of about 600 C. and maintained at this temperature for several hours. Since the reaction is not complete, the material is removed from the enclosure, ground to provide in a dry box, pressed into pellets and heated to a temperature of about 1400 C. in a sealed crucible. The resulting product is used as the evaporant of this invention.

The use of the 1:1 trivalent rare earth material is based on the theory that the system Ln Ln A has complete solid solubility. It is therefore possible to diffuse a small quantity of the electroding material into the crystal and not destroy its structure and at the same time obtain good electrical or ohmic contact.

In operation, ohmic contacts are affixed to the ferromagnetic semiconductors as follows: the ferromagnetic semiconductor crystal is placed on a substrate, masked and then positioned in a conventional electron beam gun evaporator so as to be about 14 inches directly above the electron beam source. The substrate is heated to and maintained at a temperature of from about 300 C. to

500 C. The 1:1 trivalent rare earth compound is then heated to temperatures in the range of about 2000 C. to 2600" C. by the electron beam gun. Evaporation of the trivalent rare earth compound is continued for about 0.5 to 1.0 minutes. It should be understood, however, that the time for evaporation varies with such parameters as the substrate to source distance and the amount of power applied. After the evaporation step the crystal with its evaporant is placed in sealed crucible, which may be made on a refractory material, together with a small amount of Eu metal, e.g., about 0.1 gram. The Eu metal serves to maintain an Eu vapor pressure 'within the crucible thereby preventing the loss of carriers from the crystal, i.e., to maintain the conductivity of the crystal. The crucibl and its charge is heated in a temperature range of from about 800 C. to about 1000 C. for about 1 hour to partially diffuse the evaporant into the crystal and to thereby establish the electroded areas. Conductive leads can then be soldered to the electroded areas with a conventional low temperature solder, such as In and its alloys etc. Where, in the formula Ln Ln A, A is oxygen the contacts are prepared in a slightly different manner. Since the 1:1 trivalent rare earth oxygen compounds are not easily prepared other trivalent rare earth chalcogenides are used as the contact compositions. For example, if the ferromagnetic semiconductor has a composition GdSe, GdS or GdTe may be used as the contact composition. While the ensuing examples indicate that Ln moieties of the semiconductor body and the 1:1 trivalent rare earth chalcogenide may be the same in preferred embodiments, that is not to say that they may not be different. For example, where the semiconductor body has the composition Eu Gd Se, i.e., Ln is Gd, the 1:1 trivalent rare earth chalcogenide may be LaS or any other rare earth chalcogenide.

The following examples are given by way of illustration and not by way of limitation.

EXAMPLE 1 A ferromagnetic semiconductor crystal having the formula Eu Gd Se is placed on a substrate and masked. The substrate and crystal is then positioned about 14 inches above the evaporant source material, GdSe, in a conventional electron beam gun evaporator. The source material is heated to a temperature in the range of about 2000 C. to about 2600 C. for about 1 minute to cause the GdSe to evaporate and deposit upon the exposed areas of the crystal. After depositing the GdSe on one surface of the crystal the crystal is turned on its opposite surface and treated in the same maner. After depositing GdSe on opposing surfaces of the crystal, the crystal is removed from the evaporator and placed in a sealed crucible together with about 0.1 gram of Eu metal. The crucible and its contents is then heated in an oven at a temperature of about 1000 C. for 1 hour after which the electroded crystal is removed and copper leads are soldered thereto.

The current voltage characteristics of the contacts were measured at several temperatures between 4.2 K. and room temperature. The figure shows the current to be linearly dependent on the voltage at 42 K. and at room temperature, a clear indication that the contacts are ohmic.

EXAMPLE 2 The method of Example 1 is repeated except that the ferromagnetic semiconductor crystal used has the formula Eu Gd O and the 1:1 trivalent chalcogenide is GdSe.

Other illustrative embodiments of the invention are given in the ensuing table.

For the following preferred embodiments, 3-16, the process of Example 1 is repeated except that the ferromagnetic smiconductor compositions and 1:1 trivalent 4 chalcogenide ohmic contact compositions as listed in the ensuing table are used.

Thus, the invention describes a process for aflixing good ohmic contacts to ferromagnetic semiconductor compositions having the general formula Ln Ln' A, where Ln is a first rare earth element, Ln is a second but different rare earth element and A is a chalcogen, with a 1:1 trivalent rare earth chalcogenide having the general formula Ln'A, where Ln is a second rare earth element and A is a chalcogen other than oxygen.

While the invention has been particular shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. A method for aflixing ohmic contacts to a ferromagnetic semiconductor body comprising the steps of:

(a) evaporating a 1:1 trivalent rare earth chalcogenide composition onto the exposed surfaces of said ferromagnetic semiconductor body, and

(b) heating said ferromagnetic semiconductor body sufficiently to cause said deposited 1:1 tn'valent rare earth chalcogenide composition to partially diffuse into said ferromagnetic semiconductor body.

2. A method according to claim 1 wherein said 1:1 trivalent rare earth chalcogenide is a compound component of said ferromagnetic semiconductor body.

3. A method according to claim 1 wherein said heat ing step is taking place at temperatures in the range of from about 800 C. to about 1000 C.

4. A method according to claim 1 wherein said ferromagnetic semiconductor body is composed of compositions having the general formula Ln Ln' A where Ln is a divalent rare earth element selected from the group consisting of Eu, Sm and Yb, Ln is a trivalent rare earth element other than Ln and is selected from La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, Lu, Y and Sc, and A is a chalcogen selected from O', S, Se and Te.

5. A method according to claim 1 wherein said 1:1 trivalent rare earth chalcogenide composition has the general formula Ln'A, where Ln is a trivalent rare earth element selected from La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Trn, Lu, Y and Sc, and A is a chalcogen selected from S, Se and Te.

6. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Gd Se and said 1:1 trivalent rare earth chalcogenide composition has the formula GdSe.

7. A method according to claim 1 wherein said ferromagnetic semiconductor composition chalcogenide has the formula Eu La Se and said 1:1 trivalent rare earth chalcogenide composition has the formula LaSe.

8. A method according to claim 1 wherein said ferro magnetic semiconductor composition has the formula Eu Gd Te and said 1:1 trivalent rare earth chalcogenide composition has the formula GdTe.

9. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Tb Se and said 1:1 trivalent rare earth chalcogenide composition has the formula TbSe.

10. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Dy Se and said 1:1 trivalent rare earth chalcogenide composition has the formula DySe.

11. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Lu Se and said 1:1 trivalent rare earth chalcogenide composition has ,the formula LuSe.

12. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Ho Se and said 1:1 trivalent rare earth chalcogenide composition has the formula HoSe.

13. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Er Se and said 1:1 trivalent rare earth chalcogenide composition 'has the formula ErSe.

14. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Tm Se and said 1:1 trivalent rare earth chalcogenide composition has the formula T mSe.

15. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Yb Se and said 1:1 trivalent rare earth chalcogenide composition has the formula YbSe.

16. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Sc Se and said 1:1 trivalent rare earth chalcogenide composition has the formula ScSe.

17. A method according to claim 1 wherein siad ferromagnetic semiconductor composition has the formula 6 Lu Ho Se and said 1:1 trivalent rare earth chalcogenide composition has the formula HoSe.

18. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu La O and said 1:1 trivalent rare earth chalcogenide composition 'has the formula LaS.

19. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu 'Ce O and said 1:1 trivalent rare earth chalcogenide composition has the formula CeSe.

20. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Nd O and said 1:1 trivalent rare earth chalcogenide composition has the formula NdS.

21. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Y O and said 1:1 rare earth chalcogenide composition has the formula YS.

22. A method according to claim 1 wherein said ferromagnetic semiconductor composition has the formula Eu Sc O and said 1:1 trivalent rare earth chalcogenide composition has the formula ScTe.

References Cited UNITED STATES PATENTS 3,376,157 4/ 1968 Guerci et a1. l17106 3,451,845 6/ 1969 Schiiler 117-201 WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R. 117--106 

